A related art power transmission device of this type is disclosed in Patent Literature 1 below. The power transmission device according to Patent Literature 1 comprises a first rotor provided with a winding and mechanically connected to an engine, a second rotor provided with magnets that form an electromagnetic coupling with the first rotor winding and that is mechanically connected to the drive axle, a stator provided with a winding that forms an electromagnetic coupling with the second rotor magnets, a slip ring electrically connected to the first rotor winding, a brush that electrically contacts the slip ring, a first inverter that controls electric power exchangeably between the battery and the stator winding, and a second inverter that exchangeably controls electric power between the battery and the first rotor winding via the slip ring and the brush. According to Patent Literature 1, power from the engine transmitted to the first rotor is transmitted to the second rotor via the electromagnetic coupling of the first rotor winding and the second rotor magnets, thereby making it possible to drive the drive axle with the engine power. Further, since electric power is made exchangeable between the battery and the first rotor winding via the second inverter, the rotational speed of the drive axle can be controlled by controlling the electric power of the first rotor winding with the second inverter. In that case, when the rotational speed of the first rotor is higher than the rotational speed of the second rotor, electric power generated by the first rotor winding is supplied to the battery side via the second inverter, and when the rotational speed of the first rotor is lower than the rotational speed of the second rotor, the electric power of the battery is supplied to the first rotor winding via the second inverter. Further, due to the electromagnetic coupling of the stator winding and the second rotor magnets, power supplied to the stator winding from the battery side via the first inverter may be used to generate power in the second rotor to drive the drive axle, which then allows control of torque transmitted to the drive axle by controlling electric power supplied to the stator winding by the first inverter.